1. Field of the Invention
The present invention relates to metal-oxide based memory devices and methods for operating and manufacturing such devices.
2. Description of Related Art
Some metal-oxides can be caused to change resistance between two or more stable resistance ranges by application of electrical pulses at levels suitable for implementation in integrated circuits, thus providing a basis for use in nonvolatile resistive random access memory RRAM. Metal-oxide based RRAM has attracted much attention because of its simple structure, high speed, low power consumption, and compatibility with standard CMOS processes.
In metal-oxide based memory, data is stored by applying energy pulses to the metal-oxide material to induce a change in resistance of the metal-oxide material between two or more resistance states. Multi-bit operation involves changing the resistance of the metal-oxide material between more than two resistance states and is desirable since the data storage density can be increased and the process costs can be reduced.
It has been reported that some metal-oxides require a “forming process” to cause a breakdown from a high resistance state to a low resistance state in order to enable the resistive switching behavior of the material. As illustrated in FIG. 1, the forming process typically involves increasing a DC voltage applied across the metal-oxide material until breakdown from a high resistance state to a low resistance state is achieved. The forming process occurs at a forming voltage (Vforming) which is often much larger than the magnitudes of a set pulse (Vset) and a reset pulse (Vreset) which are then applied to induce a resistive state change in the metal-oxide material. The relatively large forming voltage (Vforming) increases the complexity of the circuitry of memory devices employing these types of metal-oxide materials. Additionally, the forming process of sweeping the DC voltage takes a relatively large time (for example greater than 60 μs), which increases the test time of the device significantly. The table below summarizes the reported forming voltage for various metal-oxides.
Metal OxideForming VoltageNiOx8.1VTiO25VCuOx16.5VZrO28.8VAl2O311V
Tungsten-oxide WOx based RRAM has been shown to exhibit good resistive switching characteristics between two or more resistance ranges without the need for a forming process. See, U.S. patent application Ser. No. 11/955,137 (now U.S. Patent Publication No. 2008/0304312) entitled “Memory Devices Having an Embedded Resistance Memory with Tungsten Compound and Manufacturing Methods”, filed 12 Dec. 2007 and incorporated by reference herein.
In order to reliably distinguish between the various resistance states, and thus properly determine the data value stored in a memory cell, it is important to maintain relatively large resistance windows between the states. Additionally, it is important to maintain a large resistance window between the highest and lowest resistance states used to represent data in order to provide an opportunity to achieve multi-bit operation.
Previous attempts for increasing the resistance window between the highest and lowest resistance states involves increasing the voltage magnitude of a reset pulse applied across the metal-oxide material. However, endurance problems have arisen using large magnitude reset pulses due to instability of the resistance of the metal-oxide material, resulting in reliability issues and possible failure of the device.
It is therefore desirable to provide metal-oxide based memory devices and methods for manufacturing and operating such devices which address the endurance problems discussed above and result in improved reliability and data storage performance of the device.